Waveguides are used in optical communication networks for the transmission and routing of optical signals. For the transmission of the optical signals over long distances, waveguides can take the form of optical fibers, thin strands of glass that are used to transfer data over distances that can span tens of kilometers without a repeater. Within the networks of long range optical fibers are signal processing nodes that contain packaged photonic and optoelectronic circuits that are used to perform various functions such as to encode, send, receive, decode, multiplex, and de-multiplex, among other optical and electrical signal processing functions, the optical signals that are delivered to these processing nodes via the optical fibers. And within the optoelectronic circuits in these processing nodes, optical signals are transmitted via free space and through short lengths of waveguide. These short lengths of waveguide are used to guide signals to a variety of small packaged devices or components that can transfer, combine, split, and route optical signals as the demands of the network require.
Routing of optical signals from the optical fibers to components on the sub mount assembly have historically been accomplished via transmission in free space, and to some extent, via planar optical waveguides on the sub mount assembly. Optical transmission in free space can require lenses to focus and direct the optical signals between components in the optical circuits and can require large spatial volumes to accommodate these lenses, which can lead to undesirably large package sizes for these optical circuits. Additionally, the transmission of the signals in free space can result in significant signal losses from uncontrolled scattering and reflection. Alternatively, planar optical waveguides offer the potential for significant reduction in optoelectronic package size. The integration and patterning of planar waveguide structures on substrates allow for the transmission and distribution of optical signals without the need for large discrete optical components. Integrated waveguide structures also allow for the formation of optical device structures, such as filters, gratings, and spot size converters, for example, directly onto the substrate.
Optoelectronic packages at signal processing nodes in optical communications networks generally include an optical sub mount assembly, which typically consists of one or more optical die (such as lasers and photodetectors), and that can include either the means for the free space transmission of optical signals or the planar waveguides and associated optical routing components, all of which are enclosed in an hermetically-sealed cavity formed by a cap and a substrate. A sub mount assembly can include, for example, a substrate or interposer, the optical routing components, and the signal-generating and signal-receiving devices and components. The planar waveguide structures are deposited and patterned to form waveguides and optical device components, or in some applications, added as discrete elements. Currently, the capability for fabricating planar waveguide structures of sufficient thickness with low stress is limited, and therefore, a need exists in the art of optoelectronic packaging for a planar waveguide structure that can be deposited onto a substrate, and from which compact and economical interposers and sub mount assemblies can be formed. Thus, there is a need in the art for a planar optical waveguide structure for transmission and routing of optical signals in photonic integrated circuits that has low optical loss, has low stress, is compact, and is economically manufacturable.